1. Field of the Invention
The present invention relates to an apparatus and a method for the anti-islanding of a power conditioning system (PCS) capable of controlling power from a solar cell array, and more particularly, to an apparatus and a method for the anti-islanding of a power conditioning system (PCS) capable of easily detecting an islanding state by using an injection signal maintained at a level smaller than that of a reference level in a grid interconnected state and increased to a level larger than a reference level in an islanding state.
2. Description of the Related Art
Generally, an islanding detection algorithm for detecting an islanding state of a distributed power supply is classified as either an active method or a passive method.
First, the active method is a method of determining islanding by controlling the output from the distributed power supply and monitoring the response. This method can readily detect the islanding of the distributed power supply but directly controls the output from the distributed power supply, thereby having an unintended effect on the grid. During islanding, to supply power by interconnecting several distributed power supplies to a power grid, even though the output from the distributed power supplies is controlled, the variations corresponding to the controlled output are not generated due to the effects of other generators, such that a case in which islanding is mistaken for an interconnecting operation may occur.
Next, the passive method is a method capable of detecting islanding using a change in several parameters of the grid during islanding. Unlike the method for detecting the active distributed power supply, the passive methods can detect islanding without having a negative effect on the grid, but it is difficult to detect islanding due to the small change in parameters in islanding when keeping a balance between power from the distributed power supply and power consumed by loads.
Generally, the active methods and the passive methods for detecting islanding in the distributed power supply, leading to accidents, will be described in more detail below.
The active method will first be described.
An example of the active method for detecting islanding in the distributed power supply may include a reactive export error detector (REED), an active frequency drift (AFD), and the like. Since these active methods for detecting islanding can rapidly and readily detect islanding in the distributed power supply, but optionally vary the output from the distributed power supply or change the structure of the grid, they may lead to unintended effects in the grid.
Further, in the case in which several generators are interconnected with the distribution grid, it may be difficult to detect islanding with the active method when grid voltage is lost. The reason is that other distributed power supplies respond to the output variations of the distributed power supply. The islanding having the single distributed power supply and the loads has large grid response characteristics in connection with the output variations of the distributed power supply. However, when there are several generators, there is a disadvantage in that the islanding of the distributed power supply may be determined as the general interconnected operation form since the response characteristics corresponding to the output variations of the distributed power supply are not generated.
Next, the passive method will be described.
When the above-mentioned active method is a method of detecting the islanding by directly controlling the output from the distributed power supply, the passive method monitors the variations in the grid parameters such as voltage, phase, frequency, reactive power, or the like, which are generated when islanding the distributed power supply, to determine whether or not the grid voltage interconnected with the distributed power supply is present.
Unlike the active method, determining the islanding of the distributed power supply using the passive method does not generate negative effects due to the variations in the output of the distributed power supply, but when keeping a balance between the power output of the distributed power supply and the consumed amount of the grid load in the islanding, makes the variations between the parameters smaller. In some cases, there is a problem in that it is difficult to detect islanding.